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Hybrid organic–inorganic perovskites have generated considerable research interest in the field of optoelectronic devices. However, there have been significantly fewer reports of their thermoelectric properties despite some promising early results. In this article, we investigate the thermoelectric properties of bismuth-doped CH3NH3PbBr3 (MAPbBr3) single crystals. The high-quality Bi-doped crystals were synthesized by inverse temperature crystallization and it was found that Bi substitutes onto the B-site of the ABX3 perovskite lattice of MAPbBr3 crystals with very little distortion of the crystal structure. Bi doping does not significantly alter the thermal conductivity but dramatically enhances the electrical conductivity of MAPbBr3, increasing the charge carrier density by more than three orders of magnitude. We obtained a negative Seebeck coefficient of −378 μV K−1 for 15% (x = 0.15) Bi-doped MAPb(1−x)BixBr3 confirming n-type doping and also measured the figure of merit, ZT. This work highlights routes towards controlled substitutional doping of halide perovskites to optimise them for thermoelectric applications.

Self-organised ferromagnetic nanowires encapsulated by multiwalled carbon nanotubes produced by CVD methods based on the thermal decomposition of ferrocene commonly contain the elemental phases: α-Fe, γ-Fe, and the carbide Fe 3 C. A continuous α-Fe nanowire and control of nanowire and nanotube length and diameter are desirable. High α-Fe nanowire content has been achieved through synthesis temperature modification, vapour flow-rate, and post-synthesis heat treatment. Length and diameter are intimately related to the self-organisational growth processes; reported approaches include regulation of the vapour supply to minimise dispersion in the nucleation process and introduction of other growth-modifying precursor elements. High vapour flow-rate produces downstream fluctuation resulting in discontinuous nanowires and diameter dispersion, or external decoration with spherical particles, results from rapid evaporation of the ferrocene precursor. We report a low vapour flow-rate and constant evaporation temperature method which achieves continuous α-Fe nanowires on the same scale as the nanotube for lengths >10 μm without the necessity of post-synthesis heat-treatment or introduction of other precursor elements. The low vapour flow-rate regime has the advantage of sustaining the intrinsic temperature gradient at the tip of the forming structure which drives the vapour feedstock to the growth front to guarantee continuous nanowire formation.

Thermoelectric energy conversion within a homogeneous conductor is governed by the charge carrier mobility and the density of electronic states, intrinsic trades-off limit efficiency. Heterogeneous conduction created by the spatial variation of disorder is considered undesirable because it is conceptually difficult and not easily reproduced. However, could it be the basis for unique, exploitable thermoelectric effects? Here we show that the heterogeneous conduction typically found in conducting polymer films can result in thermoelectricity governed by only quantum mechanical tunneling of charge carriers through nanoscale Coulomb barriers at order-disorder boundaries. We conclude this to be the basis of a new category of thermoelectric system centered on heterogeneous conductors or conducting networks. We show that thermoelectricity of this origin can be engineered without doping or intrinsic trades-off. Our findings point to a wide range of alternatives to the doped inorganic semiconductor paradigm.

A simple wet chemical method involving only sonication in aqueous GdCl3 solution was used for surface functionalization of iron-filled multiwalled carbon nanotubes with gadolinium. Functional groups on the sidewalls produced by the sonication provide active nucleation sites for the loading of Gd3+ ions. Characterization by EPR, EELS, and HRTEM confirmed the presence of Gd3+ ions on the sidewall surface. The room temperature ferromagnetic properties of the encapsulated iron nanowire, saturation magnetization of 40 emu/g and coercivity 600 Oe, were maintained after surface functionalization. Heating functionality in an alternating applied magnetic field was quantified through the measurement of specific absorption rate: 50 W/gFe at magnetic field strength 8 kA/m and frequency of 696 kHz. These results point to candidacy for dual-functioning MRI imaging and magnetic hyperthermia structures for cancer therapy.

Carbon nanotubes filled with continuous crystalline nanowires of nanometre-scale diameter and micrometre-scale length of the ferromagnetic phase α-Fe were produced with a new chemical vapour deposition method. We report a new two-stage approach, a perturbed-vapour method of synthesis followed by a post-synthesis heat treatment that produces multiwall carbon nanotubes filled with at least 19 micrometre-length nanowires of α-Fe. Previously reported synthesis routes use steady-state conditions to guarantee nanowire continuity but result only in small (less than one-micrometre length) nanowires comprising isolated or mixed phases of either α-Fe, Fe 3 C, or γ-Fe. Here flower-like clusters of carbon nanotubes continuously filled with α-Fe were produced by perturbation of a laminar ferrocene (Fe(C 5 H 5 ) 2 ) vapour flow in a conventional horizontal chemical vapour deposition reactor. Single-phase filling was achieved by a post-synthesis annealing at 500 °C for 15 h in Ar flow. Electron microscopy studies revealed the high quality of the structural integrity of both nanotubes and encapsulated nanowires. These nanostructures possess a high coercivity of 580 Oe and a very high saturation magnetization of 189.5 emu/g comparable with bulk α-Fe.

We report a new chemical vapor synthesis method that exploits random fluctuations in the viscous boundary layer between a laminar vapor flow and a surface to yield a not previously observed product: radial ferromagnetically filled-carbon-nanotube structures departing from a central particle. The filling of the nanotube capillary is continuous over a scale much greater than that which can be achieved by conventional CVD. This is a simple method which does not require ultra-fine control of process parameters or highly-engineered reactor components in which a single, self-organized, ordered product is formed in randomly fluctuating vapor in the boundary layer by vapor-, liquid-, solid-phase self-organization. These fluctuations create the thermodynamic conditions for formation of the central particle in the vapor which in turn defines the spherically symmetric diffusion gradient that initiates the radial growth. The subsequent radial growth is driven by the supply of vapor feedstock by local diffusion gradients created by endothermic graphitic carbon formation at the vapor-facing tips of the individual nanotubes and is halted by contact with the surface. The radial structures are the dominant product and the reaction conditions are self-sustaining. We argue that the method has potential for scalable production of metal-carbon nanostructures with other unusual morphologies.

The electronic and optical properties of nitrogen doped tetrahedral amorphous carbon (ta-C:N) are compared with those of a new form of carbon ta-C film in which nanotube regions are included (ta-C:(NT)). It is found that nanotubes in ta-C inhibit the N doping effect in ta-C and leads to a reduction in conductivity. The results point to the nanotube regions acting as electronically isolated graphitic regions in ta-C. The refractive index in ta-C:(NT) is found to have the opposite dependency on wavelength to that of ta-C. It is proposed that the refractive index can be used as a signature for the presence of nanotubes and other large graphitic regions in ta-C.

The study of potassium-oxygen and cesium-oxygen graphite intercalation compounds (GICs) first synthesized by the authors is presented. It is shown that these GICs are formed in reactions of graphite with alkali metal/alkali Superoxide mixtures or with alkali suboxide (in the case of cesium). Stage 1 and 2 potassium-oxygen GICs Jmterplanar distance 8.44 A), and stages 1, 2 and 5 cesium-oxygen GICs (interplanar distance 9.84 A) were isolated. X-ray diffraction, XPS, TEM electron diffraction and chemical analysis were used to characterize the content and constitution of the GICs synthesized. Metal-like temperature dependence of c -axis resistivity was obtained in the 1.5–300 K range where the resistivity values at room temperature are 0.07, 0.81, 2.6 and 0.089 Ωcm for KO stagel, KO stage 2, CsO stage 1 and CsO stage 2 compounds, respectively. All the GICs exhibit low spin susceptibility (order of 10 −7 emu/g) which suggests the absence of magnetic species O 2 − .

We have observed angular dependent magnetoresistance oscillations (ADMRO) in the out-of-plane resistivity of low-stage SbCl 5 -intercalated graphite. This new phenomenon is a semi-classical geometric resonance effect inherent in quasi two-dimensional systems whose Fermi surface is weakly corrugated in the k z -direction. Typically, ADMRO is characterized by peak positions that are periodic in tan θ. The peaks in ϱ zz occur when the average of the k z -component of electron group velocity around its extended orbit is zero. Significant deviations from ‘standard’ behaviour are found. The implications for electronic structure are discussed.

Structures of carbon nanotube/ligand complexes were studied by flow linear dichroism (the differential absorption of light polarized parallel and perpendicular to the flow orientation direction) with the aim of establishing linear dichroism as a technique to study such systems. Anthracene, naphthalene, and DNA were chosen as ligands, and the potential for flow linear dichroism to probe ligands noncovalently (as well as covalently) bound to single-walled nanotubes is reported. Linear dichroism enables the determination of approximate orientations of the ligands on the carbon nanotubes.

Field emission properties of B-doped carbon nanotubes are investigated from both theoretical and experimental standpoints. Using tight-binding and ab initio calculations, it is observed that B-saturating tip edges of carbon nanotubes induce the presence of large peaks within the local density of states (LDOS) located in an energy region close to the Fermi level (Ef). These localized states suggest a field emission enhancement for the B-doped tubes. In addition, ab initio theoretical results indicate that the work function for B-doped tubes is 1.7 eV lower when compared to pure carbon-terminated nanotubes. Experimentally, it is found that B-doped tubes, which are produced by arc discharge techniques and contain B mainly at the tips, exhibit stable electron field emission at lower turn-on voltages (1.4 V/mum) when compared to pure single- and multiwalled carbon nanotubes (2.8 and 3.0 V/mum, respectively) measured under the same conditions. We strongly believe our results will bring new insights in the fabrication of stable field emission sources.

We report high electrical conductance enhancement in sparse single-walled carbon nanotube networks by decoration with Au nanoparticles. The optimized hybrid network exhibited a sheet resistance of 650 Ω sq(-1), 1/1500 of the resistance of the host undecorated network, with a negligible optical transmission penalty (90% transmittance at 550 nm wavelength). The electrical transport at room temperature in the host and decorated networks was dominated by two-dimensional variable range hopping. The high conductance enhancement was due to positive charge transfer from the decorating Au nanoparticles in intimate contact with the host network causing a Fermi energy shift into the high density of states at a van Hove singularity and enhanced electron delocalization relative to the host network which beneficially modifies the hopping parameters in such a way that the network behaves as an integral whole. The effect is most pronounced when the nanoparticle diameter is comparable to the electron mean free path in the bulk material at room temperature and there is minimum nanoparticle agglomeration. For higher than optimal values of nanoparticle coverage or nanoparticle diameter, the conductance enhancement is countered by metallic inclusions in the current pathways that are of higher resistance than the variable range hopping-controlled elements.

DC magnetoresistive effects were observed in above-percolation-threshold loaded Fe-filled carbon nanotube/polyurethane-urea composite samples. A phenomenological model is derived from interpretation of resistance relaxation for a range of axial strains. The large instantaneous magnetoresistance of + 90% observed at low axial strain was a result of conduction pathway breaking caused by preferential orientation of the conducting nanotubes perpendicular to the axial current flow: a result of the magnetic torque experienced by the ferromagnetic nanotube core. At large strain the observed large instantaneous change in resistance of - 90% resulted from voltage-driven relaxation in the conducting nanotube network. At high axial strain the competition between voltage-driven relaxation and a magnetic torque gave rise to an oscillatory component of resistance relaxation.

In this study, thermoplastic polyurethane (TPU) fibres containing multi-walled carbon nanotubes (MWNTs) and fabricated via an extrusion process were demonstrated to possess a tuneable level of electrical conductivity. A simple approach based on the time–temperature superposition applied to the electrical conductivity of carbon nanotube (CNT) percolating in a thermoplastic polyurethane (TPU) melt was also developed to predict the conductivity of the nanocomposite fibres. The observation of Arrhenius dependence of zero-shear viscosity and the assumption of simple inverse proportionality between the variation of conductivity, due to network formation, and viscosity allow a universal plot of time variation of conductivity to be composed, which is able to predict the conductivity of the extruded fibres. The same TPU/CNT fibres were also demonstrated to possess good strain sensing abilities, which makes them good candidates for applications in smart textiles.

A correspondence between Fermi surface parameters determined by semiclassical angular dependent magnetoresistance oscillations (ADMRO) and those by the Shubnikov-de Haas effect in stage-2 SbCl 5 -intercalated graphite is demonstrated. The wavevector underlying most robust ADMRO feature and strongest branch of the Shubnikov-de Haas spectrum is the radius of lower valence band primitive cylinder of the Blinowski-Rigaux model.